1. Field of the Invention
This invention relates to a polyaspartic acid excelling in biodegradability and a method for the production thereof.
2. Description of the Related Art:
Conventionally polyaspartic acid has been produced by the dehydrating method which comprises heating solid aspartic acid thereby removing water therefrom. Methods which comprise heating solid aspartic acid in the presence of an acid catalyst (U.S. Pat. No. 5,688,902, U.S. Pat. No. 5,457,176, and U.S. Pat. No. 5,830,985) or in the absence of a catalyst (U.S. Pat. No. 5,391,764 and U.S. Pat. No. 5,319,145) by using a varying drying machine, a kneading and heating device, or an oven in the neighborhood of 200xc2x0 C. thereby dehydrating and condensing the solid aspartic acid, heating solid aspartic acid in an organic solvent of a high boiling point thereby removing the water of condensation by azeotropy (U.S. Pat. No. 5,380,817, U.S. Pat. No. 5,484,945, and U.S. Pat. No. 5,756,595), and the like have been heretofore proposed for the production of polyaspartic acid.
For the purpose of polymerizing aspartic acid by using sulfuric acid, phosphoric acid, or boric acid as an acid catalyst, methods which comprises continuing the polymerization while removing the water of polymerization, and using water for aiding in uniformly mixing the acid catalyst with the aspartic acid have been disclosed. These methods, however, produce a substantially dried solid polymer by removing the water of polymerization by decompression, exposure to a stream of nitrogen, or azeotropy notwithstanding the relevant reactions proceed in their initial stages in the presence of water.
In these methods the solid aspartic acid tends to form a bulk substance during the course of polymerization and, therefore, it needs to perform such actions as disintegration on the bulk substance during the course of polymerization. Particularly when the acid catalyst is used, the polymerization forms a bulk substance like candy, thus this bulk substance must be disintegrated.
Further, the material derived from aspartic acid by heating in these methods is a solid polysuccinimide which is an intermediate of polyaspartic acid. The polysuccinimide is hydrolyzed with a base to produce a polyaspartic acid salt.
It is an object of this invention to obtain directly an acid type polyaspartic acid by a method which avoids forming a bulk substance in producing the polyaspartic acid by condensing an amino acid having aspartic acid essentially.
It is further an object of this invention to provide a novel polyaspartic acid.
The conventional method consists in performing the relevant polymerization while removing the water of polymerization. Unexpectedly it has been found that when an amino acid having aspartic acid as an essential component is suspended in water and heated, polyaspartic acid is formed even in the presence of water, the polymer yield exceeds 80%, the formed polymer is obtained in a state dissolved in water, and the polymer is an acid type polyaspartic acid polymer. As a result, this invention has been achieved.
The object of this invention is accomplished by a method for the production of a polyaspartic acid characterized by the steps of mixing an amino acid having aspartic acid as an essential component with water and heating the resultant mixture.
Further, the object of this invention is accomplished by a polyaspartic acid having such biodegradability that the ratio of biodegradation per 7 days of BOD is not less than 45%.
By the method of production according to this invention, a polyaspartic acid (containing an amino acid having polyaspartic acid as an essential component) having an excellent biodegradability can be obtained by an extremely simple procedure.
The polyaspartic acid of this invention is excellent particularly in biodegradability.
Since the polyaspartic acid of this invention excels in biodegradability, it is suitable as detergent additives, dispersion stabilizers, scale preventing agents, humectants, and fertilizers.
The other object of this invention is accomplished by a polyaspartic acid obtained by mixing an amino acid having aspartic acid as an essential component with water and then heating the resultant mixture.
The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments.
The term xe2x80x9cpolyaspartic acidxe2x80x9d as used herein also embraces salts of polyaspartic acid. The counterions of polyaspartic acid include, but not limited to those below, for example alkali metals and alkaline earth metals, specifically sodium, potassium, magnesium, calcium, strontium, and ammonium cations.
This invention uses, as the raw material, an amino acid having aspartic acid as an essential component. The aspartic acid can be used in the form of L-, D-, or a mixture thereof. The aspartic acid may additionally incorporate therein one or more other amino acids in an amount up to 99% by weight, preferably 5 to 99% by weight, based on the weight of the aspartic acid. Suitable examples of the other amino acids may include glycin, alanine, asparagine, glutamic acid, lysine, valine, leucine, isoleucine, phenylalanine, methionine, histidine, proline, serine, threonine, and cysteine. Among other amino acids mentioned above, L-glutamic acid, L-alanine, and L-lysine prove particularly preferable because they are inexpensive and the polymers produced have high water solubility.
In this invention, polyaspartic acid can be produced by heating an amino acid having aspartic acid as an essential component (hereinafter occasionally referred to simply as xe2x80x9caspartic acidxe2x80x9d) in the presence of water.
The amount of the water to be used in the polymerization, though not particularly limited, generally has a lower limit of not less than 0.2 times, preferably not less than 0.5 times, and more preferably not less than 1 times and an upper limit of not more than 40 times, preferably not more than 20 times, and more preferably not more than 10 times the weight of the amino acid having aspartic acid as a main component. The amount of the water is preferred to be in the range of 0.2 to 40 times, especially 0.5 to 20 times, based on the weight of the amino acid having aspartic acid as the main component. If the amount is less than 0.2 times, the shortage will be at a disadvantage in suffering the resultant polymer to assume a bulk form. If this amount exceeds 40 times, the excess will be at a disadvantage in suffering the aqueous solution of the produced polymer to have an unduly low concentration and rendering the polymerization uneconomical.
The heating temperature in the polymerization, though not particularly limited, generally has a lower limit of not less than 140xc2x0 C., preferably not less than 150xc2x0 C., and more preferably not less than 160 C. and an upper limit of not more than 300xc2x0 C., preferably not more than 250xc2x0 C., and more preferably not more than 220xc2x0 C. The heating temperature is preferred to be in the range of 140 to 300xc2x0 C., especially 150 to 250xc2x0 C. If the heating temperature is less than 100xc2x0 C., the shortage will be at a disadvantage in lowering the yield of polymer. Conversely, if it exceeds 300xc2x0 C., the excess will be at a disadvantage in impairing the biodegradability, one of the characteristics of the produced polymer.
The reaction is generally performed in a closed vessel proof against pressure, therefore, preferred to have an inner pressure exceeding 0.3 MPa, preferably in the range of 0.5-4 MPa.
An acid catalyst is preferably added in the polymerization of an amino acid. Commendably, an organic or inorganic acid having a lower pKa value than that of aspartic acid is used as the acid catalyst. Suitable examples of the inorganic acid may include phosphoric acids such as orthophosphoric, metaphosphoric, and polyphosphoric acids, phosphonic acids, phosphinic acids, sulfuric acid, sulfurous acid, pyrosulfuric acid, boric acid, and trifluoromethane sulfonic acid. Suitable examples of the organic acid may include acidic phosphoric esters such as methyl phosphoric, ethyl phosphoric, and phenyl phosphoric acids, phosphonic esters such as methyl phosphonic, ethyl phosphonic, butyl phosphoric, lauryl phosphonic, stearyl phosphonic, and phenyl phosphonic acids, and methane sulfonic acid and paratoluene sulfonic acid. The hydrogen salts of these acids are similarly usable. Suitable examples of the hydrogen salts of acid may include hydrogen phosphates such as MH2PO4 and M2HPO4 (wherein M denotes sodium, potassium, or calcium) and hydrogen sulfates such as potassium hydrogen sulfate, sodium hydrogen sulfate, nitrosyl hydrogen sulfate, and hydrazinium hydrogen sulfate. Among other compounds enumerated above, inorganic acids are advantageous and sulfuric acid, phosphoric acid, and boric acid are particularly advantageous.
The amount of the acid catalyst to be used has a lower limit of not less than 0.001 times, preferably not less than 0.01 times, and more preferably not less than 0.05 times and an upper limit of not more than 10 times, preferably not more than 2 times, and more preferably not more than 1 times, of the weight of the amino acid having aspartic acid as an essential component thereof. The amount of the acid catalyst is preferred to be in the range of 0.001 to 10 times, especially 0.01 to 2 times, based on the weight of the amino acid having aspartic acid as the main component. If this amount is less than 0.001 times, the shortage will be at a disadvantage in preventing the acid catalyst from fulfilling the effect thereof. Conversely, if this amount exceeds 10 times, the excess will be at a disadvantage in suffering the aqueous solution of the produced polymer to contain the unaltered acid catalyst in a large amount and consequently entailing an extra cost for disposing the residual acid catalyst.
The polymerization may be effected batchwise or continuously. In the batch case, a reactor such as a stirring vessel or a horizontal mixing device which can withstand the steam pressure at the polymerization temperature may be used. Then, in the continuous case, a method which comprises continuously feeding the reactor usable for the batch polymerization with a slurry of aspartic acid as the raw material, allowing the slurry to remain therein for a prescribed duration, and thereafter continuously removing the produced polymer from the reactor or comprises effecting the polymerization using a varying heat exchanger of a single plate, spiral, shell-and-tube, or plate type, i.e. a method resorting to a system which can withstand the steam pressure at the polymerization temperature and can continue the relevant heating operation under a condition exceeding the steam pressure for a prescribed duration of time, can be properly used. Among other reactors suggested above, particularly a line mixer proves advantageous. The line mixer can be continuously used for polymerization, when adapted to be pressed by means of a back-pressure valve and to be heated with a jacket or by immersion in a heat medium bed.
The single-tube type heat exchanger, for example, utilizes a device which comprises a unit for heating a coiled stainless steel pipe and a unit for cooling the coiled stainless steel pipe combined in series connection and a back-pressure valve incorporated so as to apply back pressure to the heating and the cooling units. The heating unit serves the purpose of furnishing the polymerization at a necessary temperature, for example, in a range of 140xc2x0 C. to 300xc2x0 C., preferably in a range of 150xc2x0 C. to 280xc2x0 C., especially in a range of 160xc2x0 C. to 260xc2x0 C. The pipe passes through a high-temperature chamber, a heating jacket, or an oil bath set at the above temperature. The retention time of the raw material liquid is generally in the range of one minute to 10 hours, preferably in the range of 2 minutes to 5 hours, and more preferably 5 minutes to 2 hours. The length of the pipe is generally not less than 0.1 m and preferably in the range of 0.5-100 m, depending on the retention time, velocity, and temperature.
Since the product occurring at a high temperature cannot be handled readily and safely in its hot state, the cooling unit serves the purpose of lowering the product temperature in the pipe to room temperature or to the neighborhood of 25xc2x0 C. by air cooling or water cooling with a water tank. The length of the pipe is generally not less than 0.1 m and preferably in the range of 0.5 to 100 m, depending on factors such as the retention time of the liquid, velocity, and temperature.
The heating unit and the cooling unit each require application of back pressure, which is generally not less than 0.8 MPa and preferably in the range of 1 to 3 MPa. The inside diameter of the pipe is generally not less than 1 mm and preferably in the range of 2 mm to 1 m, depending on factors such as the retention time of the liquid and velocity. A mixture of the amino acid having aspartic acid as an essential component with water is fed as pressed by a pump such as a slurry pump. The feed rate of this mixture is not particularly limited but is only required to fall in a range in which the conditions mentioned above are satisfied. Generally, this range is 1 ml/minute-10 liter/minute. These conditions can be applied to operations using a shell-and-tube type or other device.
When the amount of water in the polymerization is small, a horizontally mixing device, for example, is particularly preferable because the viscosity tends to increase. Though the removal of water is not particularly required in the polymerization, the water may be removed partly in an amount up to 0.2 times the weight of the amino acid having aspartic acid as a main component when the polymer has a low concentration or it is needed to concentrate. When the amount of water is unduly small, the mixture under treatment turns into a strong bulk substance. Unduly increasing the concentration is unfavorable from the operational point of view.
An acid type polyaspartic acid polymer is produced in the form of an aqueous solution by heating the amino acid having aspartic acid as a main component in the presence of water according to the method of this invention. When an acid catalyst is used in this case, the produced polymer gains in biodegradability.
The polyaspartic acid obtained by the use of the acid catalyst is preferred to exhibit biodegradability such that the ratio of biodegradation on the 7 days after the start of test for BOD is not less than 45%, preferably not less than 47%, most preferably not less than 50%.
It is generally held that the relevant polymer is readily decomposable when the ratio of biodegradation is not less than 60% and is completely decomposed when this ratio is about 70%. The polyaspartic acid produced by using the acid catalyst according to this invention is characterized by exhibiting very satisfactory initial decomposability and having not less than about half thereof complete decomposition in one week after a decomposition test.
The weight average molecular weight of the produced polyaspartic acid is generally in the range of 2000-3800.
The polymer produced by the above method remains in the form of viscosity to an aqueous solution of low viscosity at high temperatures and, when cooled, assumes the state of candy like paste to an aqueous solution. The polymer produced in this case, when necessary, may be converted into a salt by being heated again or neutralized.
The polymer produced in the form of an aqueous solution, for example, may be separated as an acid type solid polyaspartic acid by a spray drying treatment, and the solidified polymer may be converted into a polysuccinimide type polymer by being heated again to a temperature in the range of 100 to 300xc2x0 C. Further, the acid type polyaspartic acid polymer, when neutralized with a base incorporated therein, may be converted into a polyaspartic acid polymer salt. Suitable examples of the base may include hydroxides of alkali metals such as sodium, potassium, and lithium and alkaline earth metals such as calcium, magnesium, and strontium; primary organic amines such as cetyl amine, pentadecyl amine, tetradecyl amine, tridecyl amine, dodecyl amine, undecyl amine, decyl amine, nonyl amine, and octyl amine; secondary organic amines such as dibutyl amine and diamyl amine; and ammonium. Among other hydroxides of alkali metals, sodium hydroxide and potassium hydroxide prove particularly advantageous.
Further, the acid type polyaspartic acid polymer produced by polymerization, when heated with a varying amine or amino acid incorporated therein, may be converted into amine or amino acid-modified polyaspartic acid polymer (polyaspartic acid derivative).
We have further discovered that the molecular weight of the polymer is increased by concentrating the aqueous solution of the produced polymer. The method of concentration is not particularly restricted but has only to be capable of decreasing the water component of the aqueous solution. Thus, a method which comprises heating the aqueous solution of the polymer thereby expelling the water component by vaporization; decompressing the aqueous solution of the polymer thereby removing the water component; or completing the relevant reaction in an autoclave and then expelling the steam from the reaction vessel at a prescribed temperature, for example, may be adopted. Specifically, in the above heating method, the aqueous solution is generally maintained at a temperature in the range of 90-160xc2x0 C. for a period in the range of 0.1-10 hours. In the above depressing method, the aqueous solution is generally maintained under a pressure in the range of 66-666 hPa for a period in the range of 0.1-10 hours. In the above steam expelling method, a procedure which comprises maintaining the aqueous solution usually at a temperature in the range of 100-160xc2x0 C. and meanwhile decreasing the internal pressure of the autoclave may be adopted. Since the molecular weight of the polymer increases substantially in proportion to the degree of concentration, the concentration is properly effected to suit the purpose for which the produced polymer is used. The concentration is preferred to be continued till the amount of the water which is removed by the concentration reaches 90%, preferably 95%, of the initially charged amount of water. After this concentration, the produced polymer may be recovered as such or, when the polymer has been solidified, may be recovered after it has been diluted with water or with a weakly basic aqueous solution. As the base for this aqueous solution, the bases mentioned above may be used.
The polymer produced in consequence of the further concentration has a weight average molecular weight generally exceeding approximately 3800 and preferably falling in the range of 4000-6500. Comparison of the molecular weights before and after the concentration shows an apparent increase of the molecular weight.
The molecular weight of a given polyaspartic acid salt can be determined by gel permeation chromatography (GPC) using a standard sample of polyethylene glycol (PEG) of a known molecular weight, for example, as a standard and a measuring instrument available from Showa Denko K.K., type xe2x80x9cShodex OHpak Column.xe2x80x9d The results of this determination are expressed as reduced to PEG.
The biodegradability of the polyaspartic acid can be determined, for example, by the modified MITI test of the OECD test guide line.
The polyaspartic acid to be manufactured in accordance with the present invention can be used for its excellent biodegradability in additive to detergents, dispersion stabilizers, scale preventing agents, humectants, and fertilizers.